Added missing link_directory(${LENSFUN_LIBRARY_DIRS}) CMake commands

this allows to use a lensfun library installed on a custom path

rtengine/CMakeLists.txt

@@ -22,6 +22,7 @@ link_directories("${PROJECT_SOURCE_DIR}/rtexif"
     ${GTHREAD_LIBRARY_DIRS}
     ${IPTCDATA_LIBRARY_DIRS}
     ${LCMS_LIBRARY_DIRS}
+    ${LENSFUN_LIBRARY_DIRS}
     )
 
 set(CAMCONSTSFILE "camconst.json")